Increasing attention in recent years has been directed to layered laminated printed wiring or circuit boards. The interconnection of active and inactive circuit components such as integrated circuits, transistors, capacitors, diodes, memory input/output devices, and other components in electronic units such as computers, communication devices, video equipment, military electronics, etc. in high densities has required the formation of printed wiring boards having two or more layers each containing conductive metallic pathways. The multi-layered laminated printed wiring boards are required to provide sufficient numbers of pathways to economically and functionally interconnect the active and inactive components in a circuit in an effective operative fashion. In the absence of multi-layered printed wiring boards such high density, highly interconnected active circuits could not be economically manufactured and the circuits could not have effective, highly efficient operating characteristics.
In the preparation of layered printed wiring boards, conductive metallic patterns or paths are formed in or on flexible or flexible, typically organic substrate sheets. Two or more sheets containing the conductive paths are assembled and fixed into an integral layered composite. The substrates are formed using typically organic resins. After the layers are assembled, the composite is typically exposed to conditions of elevated temperature and pressure that results in curing the resin component into a hard cohesive mass separating and protecting the metallic patterns.
Many bonding resin systems are typically used in the art including epoxy, polyurethane, and other thermosetting resin systems. Such systems have chemically reactive groups, such as amines, hydroxyls, cyanates, epoxies, and others, that can interact physically and chemically with the metals in the conductive pattern in each layer of the printed wiring board. Such chemical interaction between the resin and the metal pattern can result in an undesirable chemical reaction or physical interaction and resulting degradation of the bond between the resin and the metal. Such an undesirable side reaction can produce free water, ammonia or other volatile by-products. Such resin degradation can substantially weaken the bond between the resin adjacent resin layers and the metallic pattern. Further, the production of an undesired volatile by-product such as ammonia or water can result in the formation of loci of high pressure gas during high temperature curing of the layered boards. The formation of high pressure gas caused by high temperature and pressure can result in a rapid delamination of the multi-layered assembly.
In order to reduce the tendency of layered printed wiring boards to delaminate during curing an inert oxide layer is commonly formed on the metallic pattern to separate active metal from the resin system and to prevent the interaction between the resins and the metal. Typically the oxide layers are formed by contacting the metal strips with active oxidizing substances such as oxyhalogen agents, peroxide, and others. Landau, U.S. Pat. No. 4,409,037 teaches such an oxiding agent.
The oxide layer must be uniform and must have sufficient depth to prevent the interaction between the metal and the resin. Typically, the visual color and color uniformity is used as a guide to the quality of the surface oxide layer. However, the formation of intact laminates having a peel strength greater than about 4-5 lb./in. width is also a guide.
In the manufacture of printed circuit boards, often a nonintegral, nonuniform oxide layer has often been observed. Such imperfect oxide layers can result in delamination since the interaction between the resin and the metal can occur in areas of little oxiding. We believe that in many instances the imperfect oxiding layers are formed as a result of the nature of the surface of the metallic patterns.
Clearly a substantial need exists in providing a process that results in uniform oxide layers in the manufacture of multi-layered printed wiring boards.